Bridging molecular and nano-scale kinetic regimes in the crystallization of icosahedral gold nanoparticles

A precise description of the nucleation and growth mechanisms of nanoparticles in solution addresses fundamental challenges, such as comparisons with classical models or establishing alternative ones. Ultimately, it should lead to a better understanding of the parameters that govern the particle size and properties. This study focuses on ultrasmall gold nanoparticles with an icosahedral structure, prepared by the reduction of HAuCl₄ with triethylsilane (TES) in a nonpolar solvent containing oleylamine (OY). The final particles have a constant size of 2 nm, regardless of the reaction rate varied by adjusting the TES concentration. A particularity of this synthesis lies in the nature of the precursor solution: the Au^III complexes are not free but form a suspension of 4 nm gold chloride clusters coordinated to OY. The reaction was monitored in situ through XAS and SAXS, enabling kinetic studies at both the molecular and nano-scales. The nucleation stage involves the initial Au^III clusters and stops upon their complete disappearance. Growth proceeds in two successive distinct stages: an initial rapid stage following first-order kinetics with respect to Au and TES and a subsequent slower stage that is zero-order for Au and first-order for TES. At the end of the 1^st stage, intermediate particles of 1.6 nm diameter, corresponding to 3-shell icosahedra, are formed. The rate-determining step of the 2^nd stage is the reduction of Au^III complexes adsorbed at the particle surface. The limited number of adsorption sites on the intermediate particles due to a dense ligand capping layer and/or the slow diffusion of TES through this layer may explain the very slow reaction rate of the 2^nd step. The relative stability of the intermediate magic size clusters covered with a dense ligand shell explains the abruptness of the transition between the 1^st and 2^nd growth stages. In this system, nucleation and growth do not follow the classical mechanism, and LaMer's postulate for monodisperse systems can be ruled out. This study highlights the critical influences of the stability of intermediate clusters, specific to magic size polyhedra, on growth kinetics. It also emphasizes the importance of precisely characterizing the initial precursor solution to accurately describe the nucleation.